1. Field of Invention
The present invention relates generally to ribbon bonding systems and, in particular, to bond heads for such systems.
2. Related Art
In the manufacture of semiconductor devices, active elements in a semiconductor device, such as drain and/or source regions in a semiconductor die, are electrically connected to other devices or electronic components, such as on a printed circuit board. Typically, the connection is made by bonding, e.g., ultrasonically bonding, a conductive wire between the two portions. Examples of electrical connections which can be made using wire bonding techniques include connections between the contact surfaces of discrete or integrated chips and the contact leads of their packages, and, in the case of hybrid circuits, the connections between inserted monolithic elements and the film circuit which contains them.
A number of bonding techniques have been developed, and one which has been particularly successful is a micro-welding technique using ultrasound. Conductive wire, in contact with the surface to which it is to be bonded, is moved rapidly in the direction of the surface to which it is to be bonded, so that its oxide layer breaks open. The wire is then subjected to pressure, and a permanent joint is created between the two materials. Motion of the wire is generated by an ultrasonic transducer excited by an ultrasonic generator to produce high-frequency mechanical vibrations.
One type of ultrasonic bonding uses a wedge bonding tool. The ultrasonic energy is directed to the wire by the wedge tool. The wire is fed through a guide at the bottom of the wedge. The wire is then pressed down with a small defined force to slightly deform the wire. Ultrasonic energy is then switched on, and the bonding process starts. During this time, the wire portion under the bond tool is deformed, primarily widened, with the actual change in shape depending on the size and the physical properties of the wire, the bond tool geometry, and the process parameter settings.
In addition to wires, flexible conductive ribbons can be used to electrically connect two bonding areas. Compared to round wires, wide and thin ribbons allow bonding larger cross sections and creating larger contact areas. Ultrasonic bonding can also be used to connect the ribbon to a bonding surface. Ribbon bonding is disclosed in commonly-owned U.S. Patent Publication No. 20040217488, filed May 2, 2003, which is hereby incorporated by reference in its entirety.
A typical bond head for ribbon bonding includes a transducer with a slot through which the ribbon passes. A clamping mechanism, such as two opposing clamp arms, holds the ribbon near the bonding area. In operation, the ribbon passes through the slot in the transducer, where it is held by the two opposing clamp arms. The bond head is brought down onto the ribbon and the bonding surface with ultrasonic energy to bond the ribbon to the bonding surface.
This type of bond head has certain disadvantages. Because the ribbon can be very wide, e.g., 80 mil or wider, the slot necessarily has to be very wide. Wide slots may require the transducer to be re-designed, especially if the slot is near the same width or greater than the diameter of the transducer cone. The ribbon can also easily rub against the slot, especially when the ribbon and slot are comparable in size. As a result, the ribbon can be damaged or subject to unnecessary wear. Another disadvantage is with the clamping mechanism. Since the ribbon passes between the two clamp arms, it is difficult for the user to access the ribbon for deep access bonding. If the faces of the clamp arms are not as wide as the ribbon, the ribbon may not be clamped securely, thereby possibly comprising the quality of the bond or damaging the ribbon.
Therefore, there is a need for a bond head that overcomes the disadvantages of conventional ribbon bond heads discussed above.